In the case where a system is constructed by combining modules according to the prior art, signal wires between modules are connected through connectors. In a mobile phone, for example, a display module or a camera module is connected to the main substrate in the main body through wires and connectors on a flexible substrate.
When the characteristic impedance becomes discontinuous through a connector, part of the signal is reflected, causing distortion in the signal that has passed through. This causes inter-symbol interference and causes such a problem that an increase in the speed of communication is hindered. In the case where the distance between the connector terminals is reduced in order to increase the number of signals or miniaturize the connectors, crosstalk between signals increases, which results in an increase in the speed of communication and miniaturization of the device being hindered.
In the case where the material for the connectors and the manufacturing process are more sophisticated in order to solve these problems, the manufacturing costs increase. In addition, it is difficult to automate the work for connecting modules with connectors, and therefore, the assembly costs increase. These hinder the price of the device from being reduced. Furthermore, there may be an incident such that a connector disengages due to vibrations while the device is being used, which lowers the reliability of the device.
The present inventor has proposed an electronic circuit for data communication between substrates or semiconductor integrated circuit chips using inductive coupling, that is to say, magnetic field coupling, through coils formed of wires on printed circuit boards (PCBs) and semiconductor integrated circuit chips (see Non-Patent Documents 1 to 3).
Meanwhile, it has also been proposed that microstrip lines or bus lines be coupled in close proximity for wireless communication of data using capacitive coupling and inductive coupling (see Patent Documents 1 and 2). Patent Document 1 discloses that differential transmission lines consisting of two transmission lines placed parallel to each other and terminated for matching with a terminal resistor can be placed parallel to each other in the same direction for wireless communication between two modules.
In addition, Patent Document 2 discloses that two microstrip lines that are provided on a ground plane with a dielectric body film in between and are terminated for matching with a terminal resistor can be used as a directional coupler for wireless communication between two modules when differential signals in a microwave band are inputted into the two microstrip lines.